1. The Field of the Invention
This invention relates to mirror structures, and more specifically, to mirror structures suitable for use in resonant cavity devices such as vertical cavity surface emitting lasers.
2. The Relevant Technology
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics. In particular, the various material systems can be tailored to produce different laser wavelengths, such as 1550 nm, 1310 nm, 850 nm, 670 nm, and so on.
FIG. 1 illustrates a conventional VCSEL device 10. As shown, an n-doped gallium arsenide (GaAs) substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is positioned on the GaAs substrate 12, and an n-type lower spacer 18 is disposed over the lower mirror stack 16. An active region 20, usually having a number of quantum wells, is formed over the lower spacer 18. A p-type top spacer 22 is then disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-type conduction layer 9, a p-type GaAs cap layer 8, and a p-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonate at a predetermined wavelength (or at a multiple thereof). As shown in FIG. 1, at least part of the top mirror stack 24 may include an annular shaped region 40 that is doped to be non-conductive, typically with a deep H+ implant. The annular shaped region 40 as shown defines a conductive annular central opening 42 that provides an electrically conductive path above a desired region of the active region 20.
During operation, an external bias causes an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The annular shaped region 40, and more specifically, the conductive central opening 42 confine the current 21 such that it flows through the desired region of the active region 20. Some of the carriers in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are good reflectors, some of the photons escape out as light 23. For top emitting devices, the top mirror 24 may be made slightly less reflective than the bottom mirror 16 to facilitate the escape of photons in an upward direction. After passing through the top mirror 24, the light 23 passes through the p-type conduction layer 9, through the p-type GaAs cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a typical VCSEL device, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-type substrate 12), different material systems can be used, operational details can be tuned for maximum performance, and additional structures, such as tunnel junctions, can be added, if desired.
Most VCSELs of practical dimensions are inherently multi (transverse) mode. Single lowest-order mode VCSELs are favored for coupling into single-mode fibers, and are advantageous for free-space and/or wavelength sensitive systems, and may even be beneficial for use in extending the bandwidth-length product of standard 50 μm and 62.5 μm GRIN multi-mode fiber. However, it has long been known that, although the short optical cavity (approximately 1λ) of the VCSEL favors single longitudinal mode emission, the multi-wavelength (approximately 10's of λ) lateral dimensions facilitate multi-transverse mode operation.
Higher-order modes typically have a greater lateral concentration of energy away from the center of the lasing cavity. Thus, the one way to force the laser to oscillate in only a lowest-order circularly symmetric mode or a few lower order modes is to make the lateral dimension of the active area small enough to prevent higher-order modes from reaching threshold. However, this necessitates lateral dimensions of less than about 5 μm for typical VCSELs. Such small areas may result in excessive resistance and push the limits obtainable from conventional fabrication methodologies. For example, and referring to FIG. 1, it is often difficult to control the deep H+ implant when forming the annular shaped current confining region 40, particularly when the implantation depth is greater than about 1 μm, where lateral straggle may become a limiting factor. Thus, control of transverse modes remains difficult for VCSELs of practical dimensions.
Rather than using a deep H+ implant to define an annular current confinement region 40, some VCSELs use a high aluminum bearing layer in the top mirror to provide oxide current confinement. Typically, a mesa is formed by etching around the VCSEL device (as taught, for example, in U.S. Pat. No. 5,493,577), after which the high aluminum bearing layer is laterally oxidized from the edge of the mesa to form an annular shaped current confinement region in the VCSEL device. Alternatively, trenches or depressions are formed to access and oxidize the high aluminum bearing layer as taught in U.S. Pat. No. 5,903,588. By controlling the time of oxidization, the size of the annular shaped current confinement region can be controlled. VCSELs fabricated using these methods are often called oxide-confined VCSELs.
While oxide-confined VCSELs are thought to be optically and electrically beneficial, they can be difficult to implement in practice. One reason for the difficulty is that the intentionally oxidized layer, or oxide aperture forming layer, usually has a high aluminum content and is sandwiched between layers having lower aluminum content, which may oxidize at considerably different rates. This can result in significant band discontinuities between the layers. These band discontinuities can detrimentally increase the electrical resistance of the structure and form a barrier to current flow. Attempts have been made to reduce these band discontinuities, but such attempts often result in a relatively thick oxide layers due to partial oxidation of the adjacent layers, which can increase the unwanted optical effects of the oxide layer or layers.
Another limitation of many oxide-confined VCSELs is that during the lateral oxidation of the high aluminum oxide aperture forming layer, the other mirror layers that have a lower aluminum concentration are also laterally oxidized to some degree but not to the same degree as the high-aluminum oxide aperture forming layer. It is believed that the lateral oxidation of the aluminum bearing layers creates crystalline defects or the like along the junction between the oxidized region and the non-oxidized region. These crystalline defects are believed to reduce the stability and/or reliability of the device.
Regardless of whether an oxide-confined DBR is provided, it is often beneficial for a DBR to be highly reflective, highly electrically conductive and have good thermal conductivity. For example, when a DBR is used in a VCSEL, it often is beneficial to have the DBR be sufficiently reflective so as to reduce optical losses to such a degree that efficient laser operation is achieved. Reflectivity is typically achieved by stacking material layers having significantly different indexes of refraction, for example, by stacking alternating layers of AlAs and GaAs. Such stacked layers can produce an optical standing wave within the DBR.
While the optical performance of stacked AlAs and GaAs is typically good, an abrupt junction between an AlAs layer and a GaAs layer is thought to form a high barrier to current flow. To reduce that barrier, the layers of AlAs and GaAs are typically joined using a transition region in which the material composition gradually changes from AlAs to GaAs. Furthermore, in most VCSELs, the DBR layers are doped to provide free carriers that reduce electrical resistance. The result is a structure that, ideally, has high reflectivity combined with both low optical absorption and low electrical resistance.
In practice, optical absorption increases with increasing electric field strength, increasing wavelength, and increasing doping levels. On the other hand, electrical resistance is relatively unaffected by electrical field strength, yet decreases with increasing doping levels. Therefore, obtaining both low optical absorption and low electrical resistance can be a challenge. That is, a conflict often exists between achieving reduced electrical resistance, by making the free carrier concentration higher, and reducing light absorption by making the free carrier concentration lower.
Additionally, the materials that form a DBR can strongly impact the thermal characteristics of the DBR. Binary phase materials, such as AlAs and GaAs, tend to have relatively good thermal conductivity. Thus, heat typically flows across AlAs and GaAs stacks relatively well. However, the transition region, which is characterized by three materials (e.g. AlGaAs), can have a significantly lower thermal conductivity. This is because the crystalline structure of the alloyed transition region tends to scatter phonons, the primary carriers of heat which tends to reduce the thermal conductivity of the structure. Because of the foregoing, many prior art DBRs can have excessive optical absorption, relatively poor thermal conductivity, and/or relatively high electrical resistance.